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Aquatic Megacauda enters the LRT alongside Castorocauda, but derived from the cynodonts, Probainognathus and Chiniquodon

Saturday, October 10, 2026 0:09

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The strong similarity
between Early Jurassic Megacauda (50 cm long, Fig 1) and Middle Jurassic Castorocauda (Fig 2) was readily observed by the authors, Li et al 2026, who considered the two to be members of the Docodonta. These are typically considered to be pre-mammals in Academia and in this paper.

Docodonta traditionally includes Docofossor, a burrowing Jurassic marsupial, in the LRT. This ‘clade’ also includes Agilodocodon and Microdocodon, both nesting as basal monotremes in the LRT. So docondonts are egg-laying, milk-producing mammals in the LRT.

Apart from these taxa, in the LRT Megacauda and Castorocauda are much more primitive, but may be convergent with docodonts in several regards.

Figure 1a. µCT scene of the Megacauda skull in dorsolateral view. Note the lack of a postorbital and lack of a postorbital process on the jugal, distinct from Castorocauda (Fig 2) and Chiniquodon (Fig 3). Also note the central ascending process of the premaxilla (yellow).

Figure 1b. GIF movie of the insitu Megacauda specimen in white light, in UV light and traced in line art from Li et al 2026.

Unfortunately
Li et al did not include the Triassic cynodonts, Probainognathus (Fig 3) and Chiniquodon (Fig 4), in their analysis. These two terrestrial taxa nest with the later platypus-like diggers + swimmers, Castorocauda and Megacauda in the large reptile tree (LRT, 2340 taxa). That gave them ten million years for these two terrestrial cynodonts to produce two aquatic descendants.

Taxon exclusion once again arises to hobble otherwise complete and exacting studies.

Figure 2. Castorocauda had a complete postorbital ring, a cynodont trait, along with the presence of post-dentary jaw bones that mark it as a non-mammalian cynodont. The nares are anterolateral with a short premaxillary ascending process. — Figure 2a. Castorocauda had a complete postorbital ring, a cynodont trait, along with the presence of post-dentary jaw bones that mark it as a non-mammalian cynodont. The nares are anterolateral with a short premaxillary ascending process.

Figure 1. Castrocauda insitu. Note the scale bars don't match. The pelvis shape is not mammalian. Five sacrals are present. — Figure 2b. Castrocauda insitu. Note the scale bars don’t match. The pelvis shape is not mammalian. Five sacrals are present.

The authors also mislabeled
the largest maxillary tooth, “unique in having three-rooted upper canine.”

I note the largest tooth is the third maxillary tooth, making it a premolar, so the number of roots is not unusual. The first maxillary tooth, the actual canine, is small and has a single root. Chiniquodon has this same dental pattern.

By convergence so do akynodont marsupials like Glironia. This identification has not yet percolated into mammal dental interpretations.

Figure 3. The skulls of basal cynodonts, Chiniquodon, Bolotridon and Thrinaxodon in dorsal view here shown to scale and close to actual size on an 72-dpi screen.

From the publicity:
“The well-preserved fossil even retains skin and fur impressions and reveals a clear body contour. It shows Megacauda bore the same throat musculoskeletal structure that enables modern mammals, including people, to swallow chewed food one bit at a time, rather than in massive gobs, and that provides babies the ability to suckle mother’s ⁠milk.”

“like modern mammals, it could breathe and feed at the same time.”

Let’s not overlook the possibility that suckling could have developed twice. If so, let’s look for the mammalian tooth replacement pattern, starting with toothless hatchlings needing to suckle, to determine that in fossils.

Presently, however, just sucking in aquatic prey, then spitting out unwanted water while the sharp teeth hold onto the prey prior to carrying, chewing and swallowing may be reason enough to have ‘the same throat structure.’

Figure 4. From many years ago, Chiniquodon had erect hind limbs and sprawling forelimbs, the first stage in parasagittal locomotion, a requirement for the invention of the diaphragm. Note the largest tooth on Chiniquodon is the third maxillary tooth. I never knew that was important until today. In Prozostrodon the largest maxillary tooth is the first tooth: the single-rooted canine.

This appears to be a novel hypothesis of interrelationships.
If not, please provide a citation so I can promote it here.

Figure 1. Prozostrodon nests in the LRT between Thrinaxodon and the Chiniquodon clade. — Figure 5. Prozostrodon nests in the LRT between Thrinaxodon and the Chiniquodon clade. Note the medial ascending process of the premaxilla, as seen in Megacauda in figure 1.

References
Li Y et al 2026. A Jurassic mammaliaform swimmer and transformation of the mammalian pharynx. Nature. https://doi.org/10.1038/s41586-026-11107-0 Read online without downloading here.

wiki/Castorocauda
wiki/Megacauda – not yet posted
wiki/Docodonta

Publicity

npr.org/2026/10/07/nx-s1-5991361/fossil-mammal-platypus-chew-swallow-suckle-earliest

science.org/content/article/was-odd-animal-world-s-first-milk-suckler

abc.net.au/news/science/2026-10-08/fossil-of-platypus-like-mammal-relative-found-in-china/107234186

sci.news/paleontology/megacauda-sungei-15117.html#google_vignette

reuters.com/science/otter-like-critter-swam-lakes-rivers-age-dinosaurs-2026-10-08

Source: https://pterosaurheresies.wordpress.com/2026/10/10/aquatic-megacauda-enters-the-lrt-alongside-castorocauda-but-derived-from-the-cynodonts-probainognathus-and-chiniquodon/

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